The present invention relates to an anisotropic optical film which changes the diffusibility of transmitted light depending on the incident light angle.
Members that have light diffusibility (light diffusion members) are used for lighting fixtures and building materials, as well as in display devices. The display devices include, for example, liquid crystal display devices (LCD) and organic electroluminescence elements (organic EL). The light diffusion mechanisms of the light diffusion members include scattering by asperity formed at the surfaces (surface scattering), scattering due to a refractive index difference between a matrix resin and microparticles dispersed therein (internal scattering), and both surface scattering and internal scattering. However, these light diffusion members generally have isotropic diffusion performance, and even when the incident light angles are somewhat changed, the diffusion characteristics of transmitted light do not vary significantly.
On the other hand, anisotropic optical films are known which are able to intensively diffuse incident light in a certain angle region and transmit incident light in the other angle region, that is, vary the linear transmitted light quantity depending on the incident light angles. As such an anisotropic optical film, an anisotropic diffusion medium is disclosed where an assembly of pillar-like cured regions all extending parallel to a predetermined direction P is formed within a resin layer composed of a cured product of a composition including a photopolymerizable compound (for example, see JP 2005-265915 A). It is to be noted that the structure of an anisotropic optical film where an aggregate of multiple pillar-like cured regions extending parallel to a predetermined direction P is formed as described in JP 2005-265915 A will be hereinafter referred to as a “columnar structure” in this specification.
In the anisotropic optical film which has the columnar structure, when there is light incident onto the film from top toward bottom, there is identical diffusion in a flow direction in the film manufacturing process (hereinafter, referred to as an “MD direction”) and in a film width direction perpendicular to the MD direction (hereinafter, referred to as a “TD direction”). More specifically, diffusion is isotropic in the anisotropic optical film which has the columnar structure. Therefore, the anisotropic optical film which has the columnar structure is unlikely to produce a rapid change in brightness or produce glare.
However, the anisotropic optical film of columnar structure has the problem of being low in linear transmittance in a non-diffusion region as an angular range of incident light for high linear transmittance, and small in the width (diffusion width) of a diffusion region as an angular range of incident light for low linear transmittance (that is, high diffusion intensity).
On the other hand, rather than the columnar structure mentioned above, the use of an anisotropic optical film where an aggregate of one or more plate-like cured regions is formed within a resin layer composed of a cured product of a composition including a photopolymerizable compound (see, for example, JP4802707 B2) as an anisotropic optical film can improve the linear transmittance in a non-diffusion region to increase the diffusion width. It is to be noted that the structure of an anisotropic optical film where an aggregate of one or more plate-like cured regions is formed as described in JP4802707 B2 will be hereinafter referred to as a “tabular structure” in this specification.
In the anisotropic optical film which has the tabular structure, when there is light incident onto the film from top toward bottom, there is a difference in diffusion between in the MD direction and in the TD direction. More specifically, diffusion is anisotropic in the anisotropic optical film which has the tabular structure. Specifically, for example, the width (diffusion width) of a diffusion region is made larger than that of the columnar structure in the MD direction, the diffusion width is made smaller than that of the columnar structure in the TD direction. Therefore, the anisotropic optical film which has the tabular structure has the problem of being likely to cause light interference and thus generate glare, for example, when the diffusion width is decreased in the TD direction, as a result of causing a rapid change in brightness in the TD direction.
In contrast, in order to solve the problems with the anisotropic optical film of columnar structure and the anisotropic optical film of tabular structure, provide favorable dependency on incident light angles for light transmission and diffusion, and expand the widths of the diffusion regions, for example, JP2012-141593 A discloses an anisotropic optical film obtained by stacking an anisotropic light diffusion layer of columnar structure (corresponding to the “column structure” in JP2012-141593 A) and an anisotropic light diffusion layer of tabular structure. Other background prior art is JP 2005-265915 A and JP 4802707 B2.